This NAVAID is subject to line-of-sight restrictions and does not penetrate mountains. What is it?

The correct choice is VOR, which stands for VHF Omnidirectional Range. VORs are ground-based radio navigational aids that help pilots determine their position and stay oriented during flight. The operation of a VOR relies on the line-of-sight propagation of radio waves, meaning that the signal can be obstructed by physical terrain such as mountains or tall buildings. If an aircraft is flying in mountainous areas, the effectiveness of a VOR can be significantly diminished due to these obstructions.

NDBs (Non-Directional Beacons), while also line-of-sight, are less affected by terrain because they operate on lower frequencies and can provide signals over longer distances and in more varied conditions. ILS (Instrument Landing System) is a precision approach system that involves both lateral and vertical guidance, and is primarily used for approaches rather than en route navigation. TACAN (Tactical Air Navigation) is also a line-of-sight navigational aid but is used primarily by military aircraft for tactical purposes and includes distance measuring capabilities.

The key aspect that distinguishes VOR in this context is its reliance on a clear line of sight, which makes it subject to restrictions in mountainous areas, thereby making it the correct answer to the question.